1. Field Of The Invention
The present invention relates generally to ground-based lightning detection systems and more particularly to such a system in which the absolute timing of lightning return strokes is calculated at the data collector with high, e.g. 100 microsecond, accuracy. The invention also relates to lightning and other data collection devices requiring absolute timing, watchdog capabilities, and enhanced diagnostic and networking capabilities.
2. Description Of Prior Art
In recent years, meteorological and other applications of lightning data have taken on increased significance. Short range forecasting of severe lightning storms is now possible using data collected from existing lightning detection networks. The accurate, real-time collection of lightning data promises benefits in many fields of human endeavor. In the utility industry, for example, real-time lightning data can be used to enhance preparation and response to storms, improve the dispatch and efficient use of manpower, identify lightning related outages, and assist in the design of effective lightning protection.
A National Lightning Detection Network has been established in the United States to build a long-term data base of the frequency of lightning flashes throughout the continental United States. It locates lightning flashes to ground and reports them in real-time. The network spots only cloud-to-ground flashes, filtering out intra cloud flashes.
The network consists of lightning direction finder installations located throughout the United States that measure the characteristics of lightning flashes. The lightning direction finder installations transmit data about the flash to an operations center where a network controller and associated equipment calculate the location and time of the lightning flashes. The operations center can archive the data for subsequent study and analysis and/or transmit the lightning information to users such as electric utility companies to assist in their operation of transmission and distribution systems, by facilitating real-time storm tracking.
Systems for the detection of lightning return strokes are known and one approach for detecting the return stroke is described in U.S. Pat. No. 4,198,599 which uses a gated magnetic direction finding technique and circuitry which detects only cloud to ground lightning return strokes. Equipment using the above technique has previously been developed by Lightning Location and Protection Co. (LLP) and is called a model 80-02 Automatic Direction-Finding System. It consists of an analog unit (ADF 80-02) and digital microprocessor controlled unit (DF 80-02). The analog unit detects when a cloud to ground lightning return stroke has occurred and signals the digital unit when such an event occurs. The digital unit digitizes the analog return stroke data from the analog unit producing an azimuth, polarity, and signal strength. When such equipment has been used in a network configuration, only the characteristics of the first return stroke and the count of the number of return strokes in a flash (known as the multiplicity) is sent to the Network Controller. The characteristics of subsequent return strokes in the flash are not sent to the Network Controller. The DF 80-02 forwards the flash packets by either a synchronous multidrop configuration or a point to point asynchronous configuration to the Network Controller.
The time that is recorded when the flash occurs is very crucial in the lightning flash location algorithm employed at the operations center of the network. A Location Processor Device, connected to the Network Controller, can only group the reported flash packets sent from the multiple remote DF 80-02 units by using a timing window. If the reported flash packets have incorrect times associated with them, erroneous flash locations can result or low detection efficiencies can occur. Flash location algorithms based on data from multiple DF 80-02s are known; U.S. Pat. No. 4,245,190 utilizes a triangulation technique and another method "An Analytical Solution to Obtain the Optimum Source Location Using Multiple Direction Finders on a Spherical Surface" (Journal of the Geophysical Research, Vol. 92, No. D9, Pages 10,877-10,886, Sept. 20, 1987) utilizes an eigen value technique.
The LLP DF 80-02 uses a free running internal clock with a resolution of 1 millisecond. In a network configuration, there is no means of synchronizing the DF 80-02 to NIST (National Institute of Standards and Technology) time or any other absolute time. If dedicated land lines are used as the medium for data communications between Direction Finders and the Network Controller, then the time reported by each Direction Finder in the network can be synchronized through a single time source. The Network Controller has the responsibility of correcting all of the times reported by each Direction Finder to coincide with the current Network time. This allows for proper grouping of the lightning flash data. Using dedicated land lines allows one to make the assumption that the time path delay between the Network Controller end and the remote Direction Finder and back is the same. Based on this assumption, only the Network Controller has to be synchronized to a known timing source for proper time synchronization of the remote Direction Finders. Accuracy of a few milliseconds is possible with this method.
However, if the medium of data transmission is through satellite communications, as is highly desirable for cost and other reasons, unknown path delays are introduced, making the previous technique for time tagging the lightning data unworkable. There is a very complex method of synchronizing remote Direction Finders when unknown path delays exist, called a self-synchronization method, which takes advantage of using the relative times of the first return stroke in the flash reported by the remote Direction Finders. The advantage of this method is that the network controller is able to synchronize the time at the individual direction finder sites, thus reducing the overall cost of having a synchronized clock at each remote Direction Finder. The disadvantage of the self-synchronization method is that it causes low detection efficiencies to occur at low flash rates or when multiple groups of Direction Finders become synchronized to different relative times in a large network configuration, thus leading to multiple counting of the same flash. Another disadvantage to this method is that absolute times are not able to be determined for the lightning return strokes. This makes it very difficult to combine lightning data from Direction Finders which are on different networks.
The currently available commercial lightning Direction Finder equipment also provides no means of message passing or file transfer between a remote Direction Finder and some other point in the Lightning Detection Network. Message passing is extremely important for field service repair personnel in a large Lightning Detection Network configuration where thousands of kilometers exist between the remote Direction Finder sites and the Network Controller.
The DF 80-02 is also limited in its diagnostic and communication capabilities. Further the existing equipment lacks any form of parameter tables which can be used to configure the operating procedure of the Direction Finder.
It is also desirable to obtain lightning return stroke characteristics of each stroke in a flash. This information is actually available at the ADF 80-02 but not reported by the DF 80-02. The inventors have found that the first return stroke in a flash does not always contain the highest peak signal strength which is directly proportional to the peak current of the stroke. The peak signal strength of all of the return strokes in the flash is needed in order to correctly determine the peak current of the flash.
In summary then, a need exists for a digital lightning direction finder unit capable of communicating more information about each return stroke of a flash to a network controller; exhibiting enhanced diagnostic and communication capabilities; having remotely configurable operational parameters; and providing absolute time tagging of lightning return strokes, with high accuracy, at remote direction finder sites, to accommodate unknown path delays in satellite or other data communications media.